1. Field of the Invention
This invention relates to an exhaust gas recirculation system of an internal combustion engine for effectively reducing HC (unburned hydrocarbons) and NOx (nitrogen oxides) in exhaust gases discharged from the engine.
2. Description of the Prior Art
It has been well known to re-introduce small amounts of exhaust gas in an exhaust passage into an intake mixture to restrain a combustion of the mixture in order to reduce the nitrogen oxides in the exhaust gases effectively.
In conventional exhaust gas recirculation systems, an exhaust manifold assembly and an intake passage are generally connected through an exhaust circulation passage including in its midway a recirculation flow control valve. The hydrocarbons in the exhaust gases are simultaneously recirculated into the intake passages so as to treat the hydrocarbons.
The concentration of the hydrocarbon in the exhaust gas discharged into the exhaust port of an internal combustion engine is not constant during one cycle of the engine. The maximum concentration of the hydrocarbon will occur immediately after opening the exhaust valve and immediately before closing the valve, because the hydrocarbon in quenched layers around the wall of a head of a combustion chamber is peeled therefrom by opening the exhaust valve to be exhausted along inner peripheral walls of the exhaust port, while the hydrocarbon in quenched layers at the cylinder wall is scratched by a raising piston to be exhausted immediately before closing the exhaust valve.
In the most of exhaust gas recirculation systems of the prior art, the exhaust gases from the manifold assemblies are continuously recirculated without considering such a variation in concentration of hydrocarbon, so that the exhaust gases having low concentrations of hydrocarbons containing completely burned exhaust gases are extracted, with the result that the highly concentrated hydrocarbons in the exhaust gases cannot be effectively reduced. In addition, the exhaust gas recirculation lowers the combustion temperature to increase the total amount of the hydrocarbon, so that exhaust after-treatment devices having large capacities such as catalyst converters are required.